1. Field
The present invention relates to a photomask manufacturing method, a photomask manufacturing apparatus, and a photomask.
2. Description of the Related Art
In recent years, in the manufacturing of a semiconductor integrated circuit device and a magnetic element, such as a magnetic head, it is required to form a very fine pattern. In connection with this requirement, various micro-fabrication techniques have been proposed.
In the case where a fine device pattern is intended to be formed on a wafer by exposure, it is necessary to use short wavelength light and an optical system having a large numerical aperture. The optical system having the large numerical aperture has a shallow depth of focus. Therefore, in the case where such an optical system is used, and a desired fine device pattern is intended to be formed on a wafer, it is necessary precisely maintain focusing for the whole surface of the wafer. However, there is slight unevenness inevitably formed on a wafer surface, and hence it is difficult to maintain such precise focusing for the whole surface of the wafer. This problem markedly appears in particular when an isolated device pattern is intended to be exposed.
When in order to form a device pattern with high dimensional accuracy on a wafer, an optical proximity effect correction (hereinafter referred to as OPC) is applied to the device pattern on a photomask, many minute level differences of several nanometers are planarly formed at the edge of a device pattern of a photomask. When the device pattern of the photomask is formed into a shape having the many minute level differences, a device pattern of a desired dimension can be formed on the wafer in combination with the effect by an assist pattern.
When many minute level differences exist at the edge of the device pattern of the photomask, correction accuracy of a defect caused at the edge of the device pattern needs to be improved in order to form the photomask. However, for example, in the case where a defect is caused at the level difference portion existing at the edge of the device pattern of the photomask, it is very difficult to correct such portion into an originally expected level difference shape, as compared with the case where a defect caused in a linear portion with no such level difference is corrected. Further, it is also difficult to check with high accuracy whether or not the correction performed to such level difference portion is appropriately reflected in the device pattern formed on the wafer.